RESEARCH ARTICLE SUMMARY
◥CELL BIOLOGY
Nuclear pores dilate and constrict in cellulo
Christian E. Zimmerli†, Matteo Allegretti†, Vasileios Rantos, Sara K. Goetz,
Agnieszka Obarska-Kosinska, Ievgeniia Zagoriy, Aliaksandr Halavatyi, Gerhard Hummer,
Julia Mahamid, Jan Kosinski, Martin Beck
INTRODUCTION:The nucleus harbors and pro-
tects the genetic information in eukaryotes. It
is surrounded by the nuclear envelope, which
separates the nucleoplasm from the cytosol. The
double-membrane system of the nuclear envelope
is physically connected to the cytoskeleton and
chromatin on either side. It senses and signals
mechanical stimuli. Nuclear pore complexes
(NPCs) mediate nucleocytoplasmic exchange.
They fuse the inner and outer nuclear mem-
branes of the nuclear envelope to form an aque-
ous central channel. Their intricate cylindrical
architecture consists of ~30 nucleoporins. The
nucleoporin scaffold embraces the membrane
fusion topology in a highly dynamic fashion.
Variations in NPC diameter have been reported,
but the physiological circumstances and the
molecular details remain unknown.
RATIONALE:To investigate the dynamics of NPC
conformation in vivo, we structurally analyzed
NPCs inSchizosaccharomyces pombecells. We
used cryo–electron tomography with subse-
quent subtomogram averaging to quantify the
diameter of NPCs in cells exposed to different
stress stimuli. We found a dilated NPC scaffold
in exponentially growing cells that constricted
under conditions of energy depletion and
hyperosmotic shock. Structural analysis in
combination with integrative structural model-
ing revealed that the NPC scaffold underwent
large-scale movements and bending during
constriction. Thereby, the volume of the cen-
tral channel was lessened to about one-half.
Reduced nucleocytoplasmic diffusion was ap-
parent. Scaffold nucleoporins do not have any
known motor activity that would allow them
to exert forces. Thus, it appears likely that
the observed dynamics of the diameter of the
NPC were the result of forces that are laterally
applied within the nuclear envelope. Under
conditions of both energy depletion and hyper-osmotic shock, cellular and nuclear volumes
were reduced such that nuclear shrinkage led
to an excess of nuclear membranes. We
therefore hypothesized that the NPC scaffold
is mechanosensitive. We surmised that a reduc-
tion of nuclear envelope membrane tension
resulted in a consequent NPC constriction into
a conformational ground state.RESULTS:We outline several predictions of a
conceptual model in which nuclear envelope
membrane tension regulates NPC diameter.
In such a scenario, nuclear volume and NPC
diameter should be dependent on each other.
The rigidity of the NPC scaffold should coun-
teract its dilation. On the basis of membrane
elastic theory, we predicted that osmotic pres-
sure in the nuclear envelope lumen and mem-
brane tension in the nuclear envelope act
equivalently. As a consequence, the distance
between the inner and outer nuclear mem-
branes should grow linearly with both. We set
out to test those predictions experimentally.
We found that nuclear shrinkage correlated
with NPC constriction. Recovery experiments
established that this behavior was reversible.
Constricted NPCs dilated when shifted back to
normal medium. Nuclear volumes recovered,
and cells remained viable. In exponentially grow-
ing cells, genetic perturbation of the NPC scaf-
fold resulted in further dilation. We segmented
the inner and outer nuclear membranes in
cryo–electron tomograms and quantified their
distance. As expected, the diameter of the NPC
directly correlated with the distance between
the inner and outer nuclear membranes.CONCLUSION:Our data strongly suggest that
the NPC scaffold is mechanosensitive and that
membrane tension regulates its diameter. The
data link the conformation of the NPC to the
mechanical status of the nuclear envelope.
This finding has implications for various cel-
lular processes during which the nucleus may
be exposed to mechanical forces. This includes
cell differentiation and migration, nuclear en-
velope maintenance in mechanically active
tissues, the import of very large cargos such as
viral capsids, metastasis, and changing os-
motic conditions. Under such conditions, nu-
clear envelope membrane tension will change
globally or locally. The resulting changes in
the diameter of the NPC may have functional
consequences that remain to be investigated
further in the future.
▪RESEARCHSCIENCEscience.org 10 DECEMBER 2021•VOL 374 ISSUE 6573 1341
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected] (J.K.);
[email protected] (M.B.)
†These authors contributed equally to this work.
Cite this article as C. E. Zimmerliet al.,Science 374 ,
eabd9776 (2021). DOI: 10.1126/science.abd9776READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abd9776Hyperosmotic
shockEnergy
depletionNuclear envelope tension
INM-ONM distance
nuclear sizeNuclear pore complex diameterOsmotically shocked cells Exponentially growing cells Energy-depleted cellsNPCs constrict under conditions of cellular energy depletion and hyperosmotic shock.CryoÐelectron
microscopy maps of different NPC conformations are shown isosurface rendered as seen from the cytosol
in exponentially growing (light gray), hyperosmotically shocked (yellow), and energy-depleted cells (blue)
(top). Nuclear membranes are shown in dark gray. Cartoons indicate the respective conformational changes.
Upon exposure to hyperosmotic shock and energy depletion, NPCs constrict to smaller central channel
diameters. The nuclear volume is decreased, and the distance between the inner and outer nuclear
membranes is reduced (bottom), which is indicative of a loss of nuclear envelope membrane tension. INM,
inner nuclear membrane; ONM, outer nuclear membrane.